As conventional methods of controlling cooling towers in a heat source system, methods disclosed in PTLs 1 to 3 are known. PTL 1 discloses a method of controlling the number of cooling towers to be started in accordance with a load ratio of a chiller and outside air wet-bulb temperature. PTL 2 discloses a method of controlling cooling water pumps and cooling tower fans based on a coefficient of performance (COP) of the heat source system. PTL 3 discloses a method wherein flow rates of cooling water that flows into cooling towers are measured, rotational speeds of fans are controlled in the respective cooling towers based on the flow rates, and the fans of the cooling towers are stopped when the flow rates of the cooling water flowing into the cooling towers are equal to or below a specified flow rate.